Roller clutches are often used in automotive transmission applications to allow one direction only relative rotation between a pair of annularly spaced, substantial coaxial races. The clutch generally has a cage, which provides the basic structural framework of the cage, and which is sized so that it can be installed by insertion into the annular space between the races. It is convenient if the cage, rollers and roller energizing springs together constitute a secure and stable structural unit that can be easily handled at installation. While the races and the rollers are almost invariably steel, as they take considerable loads, there are many advantages to making the cage of plastic. Among the advantages of plastic cages are light weight, cost and ease of manufacture. The cage may be molded in one piece and, if designed in accordance with the principals discussed in U.S. Pat. No. 3,917,036 to Johnson et al, assigned to the assignee of the present invention, may even be by pass molded, that is, molded by only two dies that part along one line. Another advantage to plastic is that it provides good wear and friction characteristics when it is desired to mold the cage with journal blocks that fit closely between the races for concentricity control.
A disadvantage of the molded one piece plastic cage, however, is that the plastic will also typically have a thermal coefficient of expansion and contraction that is different from the steel. The plastic will expand more than the steel races with an increase from the nominal temperature, as well as shrink and contract more with a temperature decrease. Since the cage is often molded as a circumferentially complete and uninterrupted unit, it will expand and contract in proportion to its diameter. Consequently, the size of the cage and the annular space within which it must fit will tend to diverge, rather than remain matched in size. This is especially a problem with concentricity control cages, where there is less tolerance or play between the cage and the races, even at the nominal temperature. With an outer cam clutch, that is, a clutch where inner arcuate surfaces of the journal blocks ride closely on the cylindrical surface of the inner race, the great problem is the differential shrinkage that occurs with a temperature drop. The temperature differential may be quite large, since the nominal or build temperature of the races and cage is approximately seventy degrees F, but the clutch may see cold startup temperatures in the winter of twenty or thirty below. Since the cage will shrink and contract more, it can tightly constrict around the cylindrical surface of the inner race and undesirably increase the frictional force that the clutch must overcome in order to turn
The problem noted above has been recognized in the art. The U.S. Pat. No. 4,570,762 to Husmann discloses a cage with a pair of parallel spaced side rails that have a plurality of connecting webs which, it is claimed, will facilitate elastic yielding of the cage to alleviate the differential thermal expansion problem. However, as is well known to those skilled in the art who have actually worked with and tested the various commercially available clutches, this particular approach has not gained wide acceptance. This is because of the fact that the side rails of the cage are still circumferentially complete and uninterrupted, and consequently very stiff. The cage as a whole, therefore, still expands and contracts in proportion to its diameter. An alternative approach to the problem that does work well is to create a cage which, instead of being molded as one circumferentially complete unit, is formed of separate, arcuate pieces that are flexibly joined together. This creates a cage which is not stiff and which accommodates itself very well to the annular space between the races. Its drawback is the relatively high manufacturing expense involved in separately molding and then assembling the separate pieces. The cage in Husmann, on the other hand can, at least theoretically, be molded more cheaply as an integral unit, although its particular design is not amenable to by pass molding.
A simpler solution to the problem of the stiffness that results from the circumferential continuity and completeness of a one piece cage has been suggested in Offenlegunshschrift DE No. 27 19 685 A1. There, a plastic cage of the concentric control, outer cam type has a plurality of axially extending, circumferentially spaced slots cut through the sides thereof, alternately extending from opposite axial sides of the cage. This creates a cage with substantially the flexibility of a separate piece cage, but one which is still, technically, one piece. The problem is that simply cutting slots into an existing one piece cage inevitably disturbs and interferes with its structural integrity, removing much of the advantage of being one piece. For example, in the cage disclosed in DE No. 27 19 685 A1, the slots are cut through the side rails and into the roller retention pockets, which are, therefore, no longer structurally complete, or as structurally sound. This could be especially harmful to the shipping and handling characteristics of the clutch, where it is typical to have the springs hold the rollers up against a roller rest surface side of the roller pocket. If the pocket is subject to shape change as the slots open and close, the springs and rollers could more easily fall out prior to installation of the clutch. Likewise, if slots are cut through the journal block portions of the cage, this would remove bearing area, and would cause the shape of the journal blocks to change with expansion and contraction of the cage, potentially affecting clutch performance after installation.